1. Field of the Invention
This invention relates to an electrohydraulic servodevice for position control with an actuator employing oil hydraulic pump driven by an electric motor rotatable in both directions.
2. Description of the Prior Art
Many methods have been devised to regulate position with oil hydraulic cylinders. FIG. 7 illustrates a control device for truing up the edge of web at a desired position in winding a web around the reel. The displacement of the edge of web 100 detected by the photohead 101 is amplified by the amplifier 102 to magnetize the moving coil 103. In response to the direction and strength of magnetization, the moving coil 103 is attracted to or repelled from the permanent magnet 104 placed inside the moving coil to reciprocate the spool 106 in the servovalve 105 for the valve operation. The pressurized oil from the hydraulic pump is directed by the servovalve 105 to work the hydraulic cylinder 107 for control of the displacement of the web 100 so as to set its edge at the specified position. The hydraulic pump is driven by the motor to supply constantly the servovalve 105 with pressurized oil.
FIG. 8 displays a device for opening or closing the valve 109 by the hydraulic cylinder 107, in which the piston reciprocates between the two positions to operate the valve 109. The control relay 108 is installed to prevent the excess action of the valve in opening and closing. The control signal rotates the motor 111 in either direction for driving the hydraulic pump 110. This method for change of rotatory direction can replace the alteration of flow direction of pressurized oil by the servovalve 105 as demonstrated in FIG. 7. Moreover, the pump 110 is driven only at the running stage of the valve 109.
FIG. 9 shows a driving device for reciprocating the hydraulic cylinder 107 with the axial plunger pump 112 which regulates the flow direction and speed of pressurized oil by adjusting the angle of the swash plate 114 with the DC motor 113 of small inertia. The device also includes the pressure-oil tank 115. A servovalve is unnecessary too in this device, which instead needs to furnish an additional power supply for the plunger pump. The power for the pump is low at the stationary stage, and becomes high only at the running stage of the cylinder.
In the Japanese Patent Application Provisional Publication No. 61-206802, a method is described which regulates with microcomputer the rotatory direction of electric motor for driving the hydraulic pump on the basis of the detected position of the piston in the hydraulic cylinder. Neither employs this device a servovalve and the motor has only to run at the stage of cylinder operation.
The device with the servovalve 105 as shown in FIG. 7 turns out energy loss, rise of oil temperature and wear of the parts, because the servovalve is requisite and the hydraulic pump must always run for supply of pressurized oil.
The device in FIG. 8, which overcomes the disadvantages of the device in FIG. 7, alternates the rotatory direction in response to the signal through relays, causing the strong impact on the device at the change of direction due to disuse of speed control element. The frequent maintenance also is required for the contact relays of switching.
The method in FIG. 9 for control of the rotatory direction and the speed of pressurized oil by the change in angle of the swash plate in the axial plunger pump leads to good performance; however, it is economically unfavorable because the DC motor 113 and the axial plunger pump 112 are expensive.
The device described in the Japanese Patent Application Provisional Publication No. 61-206802 has difficulty in accurate control of the position. The rotatory direction, speed and stationary stage of the electric motor are controlled by detection of displacement and speed of the piston in the hydraulic cylinder, where the speed has to be derived with differentiation of the displacement. It thus becomes probable in this method that the speed derived is not accurate enough for the control because play and inertia in the linking result in the delay between the displacement detected by the displacement detector and the rotatory speed of the electric motor.
In any method for direction control of hydraulic pump employed in FIGS. 8 and 9 and the Japanese Patent Application Provisional Publication No. 61-206802, the time delay in the starting of the cylinder by increase in the oil pressure is inevitable due to the inertia in electric motor and hydraulic pump and the compressibility of oil, when a small variation in the control signal (deviation of a measured value from a desired value) from zero point, at which the pump is in stationary stage, turns the motor to start the pump. This time delay causes the delay in control as well as the tendency in instability. For such a small deviation around zero point, therefore, the output higher than a certain level is desirable to be given to the control circuit of the pump-driving motor.
On the other hand, a large flow rate in the hydraulic circuit in response to a large deviation yields large pressure loss due to the tube resistance to decrease the oil pressure imposed on the piston in the cylinder. The gain has to be set high for quick response and large driving force against large deviations, leading in turn to instability for small deviations. In fact, a small flow rate in response to a small deviation leads to small pressure loss in the tube to impose high oil pressure on the piston, resulting in the large loop-gain for the total system. Elimination of this deficiency is achieved in a device with servovalve as shown in FIG. 7, in which the servovalve has a nonlinear property to adjust the loop gain.
In general, the controller performs effectively with small gain for small load, which makes the loop gain large, and with large gain for large load, respectively. A few special types of controllers recently become commercially available. A PID controller can change the PID parameter according to the value of input signal. Some controllers are capable of automatic alteration of parameter such as automatic selection of the parameter separately against variation by the disturbance and against change in the set point, and automatic tuning of the parameter using AI (artificial intelligence). The effective control can be attained for the electrohydraulic servodevice with such a controller. These controllers, however, are complex and expensive.